1. Field of the Invention
This invention generally relates to the field of glass compositions and devices for optical amplification.
2. Description of the Related Art
In optical telecommunications networks, high bandwidth is desired for applications such as the Internet, video on demand, and videophone. In many optical communications systems, optical signals having wavelengths in the range 1530-1560 nanometers (nm) are utilized. This wavelength range corresponds to the xe2x80x9cC-bandxe2x80x9d in telecommunications. This wavelength range also corresponds to a minimum attenuation region for silica and silica-based fibers.
Optical amplifiers are utilized to amplify the optical signals in those wavelength regions. Conventional optical amplifiers for telecommunications include erbium (Er)-doped silicate glass. The Er-doped silicate glass optical amplifier operates in the C-band and can also amplify optical signals in the 1570 nm-1620 nm range (also referred to as the L-band).
The ever-increasing demand for bandwidth has filled the erbium C-band, and is beginning to fill the L-band. In order to increase optical bandwidth, more wavelengths will need to be transmitted. One wavelength range of interest is the 1460 nm-1530 nm wavelength band, often referred to as the xe2x80x9cS-band.xe2x80x9d However, this wavelength band is outside of the Er-based material amplification range.
Within the 1460 nm-1530 nm wavelength band, trivalent thulium (Tm3+) has an emission band centered at about 1470 nm. As shown in the Tm3+ energy diagram of FIG. 1, the 3H4xe2x80x943F4 transition in Tm3+ corresponds to an emission at about 1470 nm. In order to generate a population in the 3H4 energy level, for example, 790 nm radiation is absorbed by the Tm3+ material, whereby ions are transferred to the 3H4 excited state from the 3H6 ground state.
Most Tm-doped silicate glasses have an excited state lifetime (for the 3H4 level) of less than 100 microseconds, due to the quenching of the upper level in silicate hosts. This short lifetime is less preferable for laser and amplification applications. Similarly, other Tm3+ hosts, such as phosphate glass and borate glass, are also less preferable because Tm3+ is quenched by the high phonon energy of these glasses as well.
An increased 3H4 excited state lifetime can be obtained with a Tm-doped host fluoride glass material, such as fluorozirconate or ZBLAN (57ZrF4-20BaF2-4LaF3-3AlF3-20NaF). The measured lifetime for the 3H4 excited state lifetime in ZBLAN is about 1.5 milliseconds. While laser action and optical amplification have been previously demonstrated in Tm-doped ZBLAN, this material is not advantageous for mass-produced optical amplifier applications because of the difficulties of processing fluoride glasses, the low glass transition temperature, and the less than desirable chemical durability of fluoride glasses, which suffer from deleterious effects when exposed to moisture. In addition, the emission linewidth in ZBLAN is narrow, limiting the bandwith of the amplifier.
Thus, there remains a need for optical amplifiers that operate in the 1460 nm-1530 mu wavelength band.
In view of the foregoing, according to one embodiment of the present invention, a composition comprises GeO2 having a concentration of at least 20 mole percent, Tm2O3 having a concentration of about 0.001 mole percent to about 2 mole percent, and Ga2O3, having a concentration of about 2 mole percent to about 40 mole percent. The composition can further include an alkaline earth metal compound selected from the group consisting of MgO, CaO, SrO, BaO, BaF2, MgF2, CaF2, SrF2, BaCl2, MgCl2, CaCl2, SrCl2, BaBr2, MgBr2, CaBr2, SrBr2, and combinations thereof, and having a non-zero concentration of less than about 40 mole percent. The composition can fturther include an alkali metal compound selected from the group consisting of Li2O, Na2O, K2O, Rb2O, Cs2O, Li2F2, Na2F2, K2F2, Rb2F2, Cs2F2, Li2Cl2, Na2Cl2, K2Cl2, Rb2Cl2, Cs2Cl2, Li2Br2, Na2Br2, K2Br2, Rb2Br2, Cs2Br2 and combinations thereof, and having a non-zero concentration of less than about 20 mole percent. The emission bandwidth and lineshape of the composition in the 1450 nm to 1530 nm range can be varied on the basis of one or more composition ratios and/or other parameters.
According to another embodiment of the present invention, an optical amplification device comprises a germanate glass material doped with Tm3+. The germanate glass material has a first surface configured to receive an optical signal having a wavelength of from about 1460 nm to about 1540 nm and a second surface configured to output an amplified optical signal. The germanate glass material can have the composition described above. The emission bandwidth of the germanate glass material can be varied based on the composition of the material. The germanate glass material can be configured as a core for an optical fiber. The optical amplification device can further include a pump source for producing an excited 3H4 state in Tm3+.